In order to meet the demand of user terminal on full coverage and high capacity of system, relay station (RS) may be used a lot in future mobile communication systems. Relevant definitions of RS are provided as follows:
RS: it is a random access node with the function of transmitting data of user terminal or control signaling between nodes, wherein to transmit data of user terminal or control signaling between nodes includes: transmitting data of user terminal or control signaling between base station (BS) and RS, between RS and RS, as well as between RS and user terminal (UT).
Transparent relay station: it does not transmit downlink synchronous channel broadcast message, system configuration broadcast message and resources allocation broadcast message.
Non-transparent relay station: it transmits its own downlink synchronous channel broadcast message, system configuration broadcast message and resources allocation broadcast message.
Access link: it is a wireless link starting or terminating at UT.
Relay link: it is a wireless link between BS and RS or between RS and RS.
Compared with BS, RS has the advantages of flexible arrangement and low cost, so that network construction cost can be saved. The introduction of the RS can expand the capacity of system, enlarge the coverage area of cell, and increase the transmission data rate.
For a more complete understanding of the relationship between RS and BS, reference is made to the following description taken in conjunction with the drawings. FIG. 1 is a schematic view of different application scenes and effects of RSs in the prior art. In practical use, the coverage area of transparent RS falls in the coverage area of BS entirely, while the coverage area of non-transparent RS may be partially or completely outside the coverage area of BS. As shown in FIG. 1, RS2 for providing hotspot coverage is a transparent RS, while RS1 for enlarging the coverage area of BS and RS3 for solving shadow effect are non-transparent RSs.
The foregoing are definitions associated with RS in the prior art, and then the random access procedure performed by the user terminal in the mobile communication system will be described hereinafter in detail.
In the mobile communication system, it is necessary to satisfy a specific transmission condition, i.e. uplink synchronization, for uplink data transmission, so that the uplink signals from different distances and/or different user terminals to arrive at base station or relay station synchronously. Uplink synchronization can bring the system a lot of advantages, such as reducing signal interference between adjacent time slots, improving the performance of the system, and simplifying the design of base station/relay station receiver. Therefore, before uplink data transmission, it is necessary for the user terminal in an asynchronous state to establish uplink synchronization with the network through a random access procedure, and then the user terminal performs uplink data transmission in the manner of uplink synchronization.
The basic mechanism of the random access solution used in 3GPP long term evolution (LTE) system is: a user terminal randomly selects one random access preamble among a set of random access preambles, and transmits the selected random access preamble on Random Access Channel (RACH), wherein the random access preamble is called Preamble hereinafter; after receiving the Preamble, the base station calculates the is difference between the actual time of arrival and expected time of arrival of the Preamble, puts this difference into a random access response as Timing Alignment (TA) and transmits the random access response to the user terminal; after receiving the random access response, the user terminal adjusts the time for transmitting uplink data based on the TA in order to establish the uplink synchronization with the network. After the above TA adjustment process is finished, the user terminal need to send its identity (ID) to the network. The network then judges whether the user terminal collides with other user terminals according to the ID transmitted by the user terminal and generates a contention resolution message to send this message to the user terminal so as to eliminate the collision thoroughly. In the LTE system, the random access procedure carried out in the scenario where the system does not include transparent relay stations is different from that carried out in the scenario where the system includes transparent relay stations. The random access procedure of the user terminal in the instance of including transparent relay station will be described in detail hereinafter.
FIG. 2 is a schematic flowchart of a random access method of user terminal in the case of the base station dominates transparent relay stations in the prior art. The said method includes following steps.
Step 201, the user terminal randomly selects one Preamble from all available Preambles, and transmits the selected Preamble on RACH.
Step 202, both the base station and the transparent relay stations listen to the RACH. If the Preamble is detected, the received power of the received Preamble is recorded and the TA corresponding to this Preamble is calculated. The base station starts a timer T while performing RACH detection, and waits for the dominated transparent relay stations to report the detected random access detection information in a time period set by the timer.
Step 203, the transparent relay stations report random access detection information of the detected Preamble to the base station by which they are dominated, wherein the random access detection information includes:    the identification information of the received Preamble, such as the index and transmitting time of the Preamble;    the received power of the received Preamble; and    the TA corresponding to the received Preamble.
Step 204, after the timer set in the step 202 is timeout, the base station selects the base station or a certain transparent relay station as a service node of the user terminal that initiates the random access procedure, according to its measurement results and the information contained in the random access detection information received from its dominated transparent relay stations.
It is well known in the prior art how a base station selects a service node, and it is unnecessary to go into details here.
Step 205, the base station transmits random access response with respect to the Preamble, and the random access response includes:    the identification information of the Preamble corresponding to the response information, such as the index and transmitting time of Preamble;    TA with respect to the Preamble corresponding to the response information; wherein if the base station selects the base station as the service node in the step 204, the TA here is the TA corresponding to the Preamble detected by the base station, if the service node selected by the base station in the step 204 is a transparent relay station, the TA here is the TA corresponding to the Preamble reported by the selected transparent relay station;    the time-frequency position information of the channel resources allocated for the subsequent uplink data transmission; and    the temporary ID for users allocated by the base station (C-RNTI, Cell Radio Network Temporary Identity, C-RNTI).
Step 206, after receiving the random access response, the user terminal determines whether the target user terminal of the random access response is the user terminal itself according to the identification information of the Preamble in the random access response, i.e. judges whether the identification information of the Preamble in the random access response is the same as the identification information of the Preamble selected by itself in the step 201. If they are identical, the transmission timing advance of the uplink signal is adjusted in accordance with the TA included in the random access response; if they are different, no operation is performed and the random access response continues to be received. There are two situations in the latter case: the user terminal determines that the Preamble transmitted itself receives no response and thus initiates the random access procedure again or gives it up; or the user terminal obtains its random access response from the continuously received random access response and continues to execute the subsequent operations.
Step 207 is executed if the service node selected in the step 204 is the base station, while step 209 is executed if the service node selected in the step 204 is a transparent relay station.
Step 207, the user terminal transmits uplink signaling, which uses uplink resources that are allocated by the base station in the step 205. The uplink signaling transmitted by the user terminal includes at least identification information of the user terminal, such as International Mobile Subscriber Identity (IMSI), or Temporary Mobile Subscriber Identity (TMST), or C-RNTI.
Step 208, the base station determines whether the user terminal collides with other user terminals according to the identification information of the user terminal transmitted by the user terminal in the step 207, and notifies the user terminal of collision detection result through a contention resolution message.
Then, the random access procedure is finished, and the user terminal formally begins the uplink data transmission service.
Step 209, the user terminal transmits uplink signaling to the transparent relay station that is selected as its service node, which uses uplink resources that are allocated by the base station in the step 205.
The uplink signaling transmitted by the user terminal at least includes identification information of the user terminal, such as IMSI, or TMSI, or C-RNTI.
to Step 210, the transparent relay station served as the service node transmits the uplink signaling received in the step 209 to the base station.
Step 211, the base station determines whether the user terminal collides with other user terminals according to the identification information of the user terminal transmitted by the user terminal in the step 209, and notifies the transparent relay station of the collision detection result through a contention resolution message.
The details of the contention resolution message have not been determined in the prior art.
Step 212, the transparent relay station transmits the contention resolution message received in the step 211 to the user terminal.
Then, the random access procedure is finished, and the user terminal formally begins the uplink data transmission service.
The disadvantages of the prior art lie in: in the current random access procedure in which the transparent relay stations participate, the base station starts a timer T while detecting the Preamble on the RACH, and will always wait for its dominated transparent relay station to report the random access detection information before the timer is timeout. The base station does not select service node for the user terminal according to its detection results and information reported by its dominated transparent relay stations until the timer T is timeout. Due to the length of the timer T and the long time taken in judging process for selecting the service node, the random access procedure of the user terminal lasts for long time.